LibGUI: Add GModelSelection to help implementing multiple-select views

Each GAbstractView now has a GModelSelection backed by a simple
HashTable<GModelIndex>. When the selection changes somehow, the view
gets notified via the notify_selection_changed() callback.

In the future it will probably make sense to move to using some kind of
ranges as the internal representation instead.

Libraries/LibGUI/GModelSelection.cpp

@@ -0,0 +1,39 @@
+#include <LibGUI/GAbstractView.h>
+#include <LibGUI/GModelSelection.h>
+
+void GModelSelection::set(const GModelIndex& index)
+{
+    ASSERT(index.is_valid());
+    if (m_indexes.size() == 1 && m_indexes.contains(index))
+        return;
+    m_indexes.clear();
+    m_indexes.set(index);
+    m_view.notify_selection_changed({});
+}
+
+void GModelSelection::add(const GModelIndex& index)
+{
+    ASSERT(index.is_valid());
+    if (m_indexes.contains(index))
+        return;
+    m_indexes.set(index);
+    m_view.notify_selection_changed({});
+}
+
+bool GModelSelection::remove(const GModelIndex& index)
+{
+    ASSERT(index.is_valid());
+    if (!m_indexes.contains(index))
+        return false;
+    m_indexes.remove(index);
+    m_view.notify_selection_changed({});
+    return true;
+}
+
+void GModelSelection::clear()
+{
+    if (m_indexes.is_empty())
+        return;
+    m_indexes.clear();
+    m_view.notify_selection_changed({});
+}